As is known, a nuclear reactor consists of thousands of vertical metal tubes supported in parallel arrays. The tubes are grouped into fuel assemblies. Each of the fuel tubes contains uranium oxide nuclear fuel or the equivalent and is sealed to contain fission products generated during the operation of the reactor. Present methods of locating assemblies with fuel tubes that have failed are very slow and usually entail placing a fuel assembly into a container in a spent fuel pool and drawing water from the container through the fuel assembly. The water drawn through the container is then monitored for detection of fission products. Such a process requires a long time, over an hour per fuel assembly. Since the power generated by a large nuclear reactor during full operation is worth a great deal of money, it will be appreciated that a quick method and apparatus for detection of faulty fuel tubes is highly desirable, since it reduces the time that the reactor must be shut down. Such faults usually occur when a hole or crack forms in the wall of a fuel tube. This permits water to enter the interior of the tube where it contacts the uranium oxide or the like, causing corrosion of the fuel tube and further damage. When the fuel is at its usual high temperature during reactor operation, the water within the rod will contact the hot fuel and will boil.